1. Field of Invention
The present invention relates to a stepping motor, and particularly relates to a stepping motor comprising a stator provided with stator yokes each having magnetic pole teeth.
2. Related Art
Heretofore, a stepping motor is configured, for example, in such a manner as shown in FIGS. 8 and 9.
That is, as shown in FIG. 8, a stepping motor 1 includes a hollow cylindrical housing 2 having an open end, a bracket 3 for closing the open end of the housing 2, a stator 4 provided along the inner circumferential surface of this housing 2, and a rotor 5 supported rotatably by bearing portions 2a and 3a provided in the housing 2 and the bracket 3 respectively.
The above-mentioned housing 2 is made of metal material and provided with the bearing portion 2a in a position near the center of the closed end surface of the housing 2.
The above mentioned bracket 3 is made of metal material and provided with the bearing portion 3a in a position near the center of the bracket 3 in the same manner as in the housing 2.
As shown in FIG. 9, the above-mentioned stator 4 is constituted by a substantially cylindrical yoke unit 6 in which four stator yokes 6a to 6d each provided with a ring-like flange portion having an outer diameter substantially equal to the inner diameter of the housing 2 so as to be stored in the housing 2 are integrally molded with a bobbin 7 of resin, and coils 8a and 8b wound on coil winding portions defined by the stator yokes 6a and 6b and by 6c and 6d respectively. The stator 4 is stored and fixedly held in the housing 2 after the coils 8a and 8b have been wound.
The above-mentioned rotor 5 is constituted by a rotating shaft 5a rotatably supported by the above-mentioned bearing portions 2a and 3a, and a hollow cylindrical rotor magnet 5b fixedly held on the rotating shaft 5a. This rotor magnet 5b is formed to have an outer diameter slightly smaller than the inner diameter of a central hole of the yoke unit 6. The rotor magnet 5b is magnetized circumferentially.
Here, the above-mentioned stator yokes 6a to 6d are configured, for example, as shown in FIG. 10.
Since all the stator yokes 6a to 6d have the same shape, only the stator yoke 6a is shown in FIG. 10.
In FIG. 10, the stator yoke 6a is made of conductive metal material, and constituted by a ring-like flange portion 6e and a large number of magnetic pole teeth 6f provided to erect vertically from an inner circumferential portion of this ring-like flange portion 6e.
The four stator yokes 6a to 6d are combined so that the stator yokes 6b and 6c are put back to back with each other, and the magnetic pole teeth 6f of one pair of the stator yokes 6a and 6b are shifted in phase by a predetermined value from those of the other pair of the stator yokes 6c and 6d. In this state, the stator yokes 6a to 6d are stored in a mold for molding the bobbin 7, so that the bobbin 7 of resin is integrally molded around the stator yokes 6a to 6d. The yoke unit 6 is completed thus.
In the illustrated case, the stepping motor 1 has a flange 9 on the closed end side of the housing 2 so as to be attached by means of screws or the like to an electronic equipment utilizing the stepping motor 1.
According to the stepping motor 1 configured thus, when a driving current is made to flow into the respective coils 8a and 8b of the stator 4, magnetic fields generated in the coils 8a and 8b interact with the magnetic field of the rotor magnet 5b through the respective stator yokes 6a to 6d of the yoke unit 6. Then, the rotor 5 is driven and rotated intermittently by the effect of the magnetic pole teeth 6f of the respective stator yokes 6a to 6d.
However, the stator yokes 6a and 6d located on the opposite ends are in tight contact with the end surface of the housing 2 or the surface of the bracket 3 while the stator yokes 6b and 6c located inside are in back-to-back contact with each other. Therefore, magnetic flux leaks out of the magnetic circuits constituted by the pair of stator yokes 6a and 6b and the pair of stator yokes 6c and 6d together with the coil 8a and 8b respectively so that magnetic interference is generated between the magnetic circuits.
Here, in the case where the stepping motor 1 is driven by so-called single-phase excitation, and when, for example, the rotor 5 stops sequentially at rotor stop positions shown by the symbols S1, S2, S3, S4 . . . with respect to the magnetic pole teeth 6f of the stator yokes 6a to 6d as shown in FIG. 11, theoretically, the magnetic center is switched as shown by the symbols A, C, B and D.
However, when the above-mentioned leakage of magnetic flux is generated, for example, at the stop position S2, the magnetic flux of the stator yoke 6c slightly leaks also to the stator yoke 6b so that the magnetic center is slightly shifted from the position C toward the position B. Accordingly, the position where the rotor 5 stops actually is also shifted to the position B side.
Similarly, at the stop position S3, the magnetic flux of the stator yoke 6b slightly leaks also to the stator yoke 6c so that the effects of excitation of the yokes 6b and 6c are canceled with each other. Accordingly, the magnetic center is slightly shifted from the position B toward the position C. Accordingly, the position where the rotor 5 stops actually is also shifted to the position C side.
In order to reduce such shift of the stop position of the rotor 5 due to the leakage of magnetic flux, heretofore, there are a method in which the dimensions of the comb-like magnetic pole teeth 6f in each phase are changed to balance the magnetic force among the respective pole teeth portions 6f; a method in which the angle with which the stator yokes 6b and 6c are combined is slightly shifted from a theoretical angle in order to correct the stop position; and so on. However, since scattering of the magnetic force among the respective magnetic pole teeth 6f changes in accordance with the combination of the condition of winding of the coils, the intensity of the magnetic force of the rotor, and so on, it is difficult to correct the displacement of the stop position perfectly.
On the other hand, in the case where the stepping motor 1 is driven by so-called two-phase excitation, and when, for example, the rotor 5 stops sequentially at rotor stop positions shown by the symbols S1, S2, S3, S4 . . . with respect to the magnetic pole teeth 6f of the stator yokes 6a to 6d as shown in FIG. 12, theoretically, the positions D and A are excited at the stop position S1 so that the magnetic center coincides with the middle position between the positions D and A, that is, the stop position S1. Similarly, the positions A and C are excited when the rotor 5 stops at the stop position S2, the positions C and B are excited at the stop position S3, and the positions B and D are excited at the stop position S4, so that the magnetic centers in the respective cases theoretically coincide with the middle positions between the excited positions, that is, the stop positions S2, S3 and S4, respectively.
However, when the above-mentioned leakage of magnetic flux is generated, for example, at the stop position S2, the magnetic force of the housing 2 is given to the stator yoke 6a so that the magnetic force in the position A becomes intensive more than that in the position C. As a result, the magnetic center is slightly shifted toward the position A. Accordingly, the position where the rotor 5 stops actually is also shifted on the position A side.
Similarly, at the stop position S4, the magnetic force of the bracket 3 is given to the stator yoke 6d so that the magnetic force in the position D becomes intensive more than that in the position B. As a result, the magnetic center is shifted toward the position D. Accordingly, the position where the rotor 5 stops actually is also shifted to the position D side.
Further, in the stepping motor 1 configured thus, the stator yokes 6a to 6d are generally made of material having a thickness t of 1.0 or 0.8 mm as shown in FIG. 13(A).
In the case where the step angle, that is, the interval between the above-mentioned stop positions is made small, if the tooth width w of each of the magnetic pole teeth 6f of the stator yokes 6a to 6d is made narrow, that is, not larger than the thickness t, not only the stator yokes 6a to 6d are difficult to be finished, but also the accuracy of finishing becomes low.
It was therefore difficult to practically manufacture a stepping motor which was small both in size and in step angle, for example, a stepping motor in which the diameter was not larger than 50 mm and the step angle was not larger than 1.8 degrees.
In addition, when the thickness t of each of the stator yokes 6a to 6d was made thinner than the tooth width w of each of the magnetic pole teeth in order to improve the accuracy of finishing, the saturated magnetic capacities of the stator yokes 6a to 6d were lowered so that the magnetic force of the stator 4 as a whole was lowered. Therefore, there was a problem that the torque of the stepping motor 1 was lowered.
Further, in order to prevent the torque of the stepping motor 1 from being lowered, it is general that the length L of each of the magnetic pole teeth is made to be, for example, 1.5 times as large as an usual length, while the tooth width w is not changed, so that the area where each of the stator yokes 6a to 6d is opposite to the rotor 5 is increased.
However, when the length L of each of the magnetic pole teeth is increased, the magnetic inertia of magnetism emitted from the respective stator yokes 6a to 6d to the rotor 5 becomes high. As a result, when excitation is changed over at a high frequency, magnetic responsibility becomes low. Therefore, there was a problem that the high-speed rotation characteristic of the stepping motor 1 deteriorated. This is because the step angle is small so that the motor rotation speed becomes too low when the stepping motor 1 is driven at a low frequency.